Telecommunications systems, cable television systems and data communication networks use optical networks to rapidly convey large amounts of information between remote points. In an optical network, information is conveyed in the form of optical signals through optical fibers. Optical networks may also include various subsystems, such as amplifiers, dispersion compensators, multiplexer/demultiplexer filters, wavelength selective switches, spectral inverters, couplers, etc. configured to perform various operations within the network.
The distance that an optical signal can be transmitted with optical amplifiers for a given data rate depends on the impairments in the transmission system. Impairments can include accumulated amplified spontaneous emission (ASE) noise, chromatic dispersion (CD), nonlinear optical effects (such as nonlinear phase noise), polarization mode dispersion, and polarization dependent loss. Digital signal processing (DSP) in coherent optical receivers may compensate for linear impairments such as CD, polarization mode dispersion and polarization dependent loss effectively. Intra-channel nonlinear impairment may also be compensated using digital back propagation in a coherent optical receiver with DSP, but it requires high computation power, which increases with optical signal bandwidth. Typically, the higher the data rate and the denser the wavelength spacing, the more susceptible the transmission system is to impairments.
Nonlinear phase noise (NLPN) may be mitigated by mid-span spectral inversion when the optical signal is transmitted across multiple spans. Mid-span spectral inversion may be achieved optically (e.g., optical phase conjugation based on optical parametric process) or electronically (e.g., Optical-Electrical-Optical (O-E-O)). Spectral inverters (SIs) may be configured to change or maintain the wavelength after spectral inversion. The accumulated CD and NLPN of an optical signal starts to be reversed after spectral inversion. Thus, to have best compensation of CD and NLPN, placement of SIs has been limited to the mid-point of a transmission link, such that the link is symmetric with respect to the SI. In real world systems, symmetric placement for SIs may not be feasible or practical.